Rear brake choices of the early Dakota were either 9" or 10". Should be lots of 9" backing plates around. Now if someone wants to part with only the backing plates would be another question. I have a Dakota rear axle here from a parts truck, but the axle is still good, not sure I would want to go through the work of pulling off the backing plates, and on top of that, I'd have to measure if they were 9" or 10" brakes.
On the other hand, if the rear axle from my 4x4 craps out, I'd pull those 9" backing plates off and send them to you. Gene